Thermoplastic polyesters, such as polyethylene terephthalate (PET), polyethylene naphthalene (PEN), and mixtures thereof, have been used extensively for packaging applications, in particular for food packaging. PET for example is widely used for producing bottles and other containers by blowmoulding a preform into its final shape.
For food preservation, oxygen diffusion from the atmosphere through one or several layers of thermoplastic polyester packaging must be slowed down and even stopped altogether to prevent it from oxidizing the food contained therein. Similarly, for containers containing carbonated or nitrogenated beverages, diffusion of carbon dioxide or nitrogen from inside to outside the container, through the container walls must be slowed down. Polymers in general have poor barrier properties against diffusion of gases such as oxygen, carbon dioxide, and nitrogen. Polyesters are no exception to this rule, and additives must be integrated into the polyester composition to enhance barrier properties. Such additives may be passive, simply hindering or blocking the passage of the gases, or they can be active scavengers, reacting with a gas such as oxygen. Several polyester compositions particularly suitable for food packaging and blowmoulded containers with enhanced barrier properties have been proposed in the prior art.
One commonly used additive is polyamide, as disclosed in US20060052504, US20080161472, US20060105129, WO200110947, US20060106192. A particularly suitable polyamide is MXD6 which represents a family of polyamides produced through polycondensation of meta-xylylene diamine (MXDA) with adipic acid and has been described as advantageous for enhancing gas barrier properties, as disclosed, e.g., in US20070260002, WO200662816, US20060226565, WO2006125823, US20060199921. MXD6 is an aliphatic polyamide resin which contains meta-xylylene groups in the molecule as shown below:

Transition metal salts, such as cobalt salts, can be added to the polyamide containing PET to catalyze and actively promote the oxidation of the polyamide polymer, thereby further enhancing the oxygen barrier characteristics of the package and thus acting as an active oxygen scavenger. The use of cobalt salts together with a polyamide is described in the foregoing references.
In EP1173508 it is proposed to add an additive comprising from 3 to about 8 hydroxylic groups to lower the level of acetaldehyde in the polymer. Such additives, however, are not described as affecting gas barrier properties of the polymer. Trifunctional or tetrafunctional comonomers such as trimellitic anhydride, trimethylol propane, pyromellitic dianhydride, pentaerythritol, and other polyester forming polyacids or diols are known branching agents for thermoplastic polyesters, as discussed e.g. in US20120199515, US20070066719, and CA2694861. Carbon black is a known additive for enhancing heating rate of a preform to blowmoulding temperature (cf. e.g., WO2008008813).
Often the presence of additives in a base polymer is detrimental to the processing and mechanical properties of the final composition. For blowmoulded containers suitable for containing beverages, impact strength is quite important, since during handling (filling, storage, transportation), dropping of such bottles may happen through accidental mishandling. Because of this drop in mechanical properties, higher amounts of polymer are often used to thicken the containers walls, thus increasing proportionally the cost in raw materials, but also increasing the technical production problems associated with injection molding: preforms with thicker walls impact cycle time negatively with longer cooling times required to solidify the preforms, with associated formation of higher crystallinity. It follows that enhancing gas barrier properties of a thermoplastic polyester may increase considerably the cost of production of a container.
The choice of materials for the preforms and, ultimately, the blowmoulded container is quite delicate because the selected materials must fulfil the sometimes contradictory requirements imposed by the processing windows of both injection moulding for the production of the preforms, and blowmoulding for the production of the containers. Issues like melt viscosity, crystallinity, mollecular weight, melt temperature, blowmoulding temperature, must be addressed very carefully when selecting a material for blowmoulding thermoplastic containers. As shown in FIG. 1, blowmoulding a container from a thermoplastic preform is a multistage process comprising injection moulding a preform, heating said preform to blowmoulding temperature and blowmoulding the heated preform in a tool to form a container or bottle.
As a first step, a preform is produced by injection moulding or, in some cases, by extrusion. The preform may be formed of a single layer or, on the contrary, comprise several layers. The various layers can be formed by separate preform elements which are inserted into each other to form a multilayer preform assembly. An alternative process consists of simultaneous or sequential injection of successive layers on top of each other to yield an integral preform.
As a second step, the preform is heated, generally in an infrared (IR) oven to a blowmoulding temperature comprised between Tg and the melting temperature, Tm, of the preform material(s). Depending on the preform geometry (thickness) and residence time in the oven, it is possible that the temperature of the preform is locally in-homogenous, but theoretically. blowmoulding temperature is comprised between Tg and Tm. Again, wall thickness of the preform influence the blowmoulding process. First, higher energy is required to heat a thick wall preform. Second, the pressure required to blow a container out of a preform increases with the wall thickness of the preform. Third, temperature gradients are more likely to happen with thick wall preforms. All these effects can considerably affect the production cost of mass produced blowmoulded containers
For injection moulding, it is also financially advantageous to lower the injection temperature, Tinj.m, in combination with lower injection pressures, Pinj.m, in terms of equipment investment and energy consumption. On the other hand, such parameters should not be optimized to the detriment of cycle time which is a serious economical factor in mass production of containers. Furthermore, the length to thickness ratio, L/T, of the injection moulding tool cavity is also of importance for the production of thin parts and thus of lighter preforms and, ultimately, lighter containers. Thicker parts may be easier to inject but longer to cool, to the extent that higher crystallinity develops at thick sections of a part, and longer cycle times are thus required. Preforms with excessive crystalinity can no longer be blowmoulded properly. The obvious answer to all the foregoing requirements is to lower the viscosity of the melt, by e.g., lowering the molecular weight of the thermoplastic polymer. This, however, is detrimental,                (a) to the mechanical properties of the final container, since a polymer of low molecular weight is generally weak, and        (b) to the crystallinity of the preform since short chains are more mobile and tend to crystallize quicker. A preform having high crystallinity may be difficult to blowmould, since blowmoulding temperature is below melt temperature of the thermoplastic preform and crystals will not stretch to the rate imposed by blowmoulding.        
For blowmoulding, it is also financially desirable to lower both blowmoulding temperature, Tblow.m, to reduce heat energy supplied per preform, and pressure, Pblow.m, as the costs associated with supplying high pressure air at a high rate during the blowmoulding operation is quite consequent. This is possible to achieve with thinner sections which require less time to heat and less energy to stretch. The wall thickness, however, is limited by the presence of certain additives and by the requirements of injection moulding, which include the use of low molecular weight polymers characterized by a low melt viscosity, known to yield, however, poor mechanical properties.
The cost of such high performance mass produced containers must be low because of its main consumer goods applications, whilst the properties thereof such as gas barrier properties and burst pressure must be optimized. The quadrature of the circle is rendered more complex yet by the sometimes contradictory process requirements during injection moulding of the preforms and blowmoudling of the containers. There therefore remains a need in the art for thermoplastic polyester compositions particularly suitable for food packaging, yielding blowmoulded containers having good barrier properties and concomitantly good mechanical properties producible cost-effectively. The present invention proposes a solution to such need.